Not Applicable
Not Applicable
The present invention is generally directed to tools and tool blanks having a composite construction including regions of differing composition and/or microstucture. The present invention is more particularly directed to cemented carbide rotary tools and tool blanks for rotary tools having a composite construction. In addition, the present invention is directed to a method for producing rotary tools and rotary tool blanks having a composite construction. The method of the present invention finds general application in the production of rotary tools and may be applied in, for example, the production of cemented carbide rotary tools used in material removal operations such as drilling, reaming, countersinking, counterboring, and end milling.
Cemented carbide rotary tools (i.e., tools driven to rotate).are commonly employed in machining operations such as, for example, drilling, reaming, countersinking, counterboring, end milling, and tapping. Such tools are conventionally of a solid monolithic construction. The manufacturing process for such tools involves consolidating metallurgical powder (comprised of particulate ceramic and binder metal) to form a compact. The compact is then sintered to form a cylindrical tool blank having a solid monolithic construction. As used herein, monolithic construction means that the tools are composed of a material, such as, for example, a cemented carbide material, having substantially the same characteristics at any working volume within the tool. Subsequent to sintering, the tool blank is appropriately machined to form the cutting edge and other features of the particular geometry of the rotary tool. Rotary tools include, for example, drills, end mills, reamers, and taps.
Rotary tools composed of cemented carbides are adapted to many industrial applications, including the cutting and shaping of materials of construction such as metals, wood, and plastics. Cemented carbide tools are industrially important because of the combination of tensile strength, wear resistance, and toughness that is characteristic of these materials. Cemented carbides materials comprise at least two phases: at least one hard ceramic component and a softer matrix of metallic binder. The hard ceramic component may be, for example, carbides of elements within groups IVB through VIB of the periodic table. A common example is tungsten carbide. The binder may be a metal or metal alloy, typically cobalt, nickel, iron or alloys of these metals. The binder xe2x80x9ccementsxe2x80x9d the ceramic component within a matrix interconnected in three dimensions. Cemented carbides may be fabricated by consolidating a metallurgical powder blend of at least one powdered ceramic component and at least one powdered binder.
The physical and chemical properties of cemented carbide materials depend in part on the individual components of the metallurgical powders used to produce the material. The properties of the cemented carbide materials are determined by, for example, the chemical composition of the ceramic component, the particle size of the ceramic component, the chemical composition of the binder, and the ratio of binder to ceramic component. By varying the components of the metallurgical powder, rotary tools such as drills and end mills can be produced with unique properties matched to specific applications.
The monolithic construction of rotary tools inherently limits their performance and range of application. As an example, FIG. 1 depicts side and end views of a twist drill 10 having a typical design used for creating and finishing holes in construction materials such as wood, metals, and plastics. The twist drill 10 includes a chisel edge 11, which makes the initial cut into the workpiece. The cutting tip 14 of the drill 10 follows the chisel edge 11 and removes most of the material as the hole is being drilled. The outer periphery 16 of the cutting tip 14 finishes the hole. During the cutting process, cutting speeds vary significantly from the center of the drill to the drill""s outer periphery. This phenomenon is shown in FIG. 2, which graphically compares cutting speeds at an inner (D1), outer (D3), and intermediate (D2) diameter on the cutting tip of a typical twist drill. In FIG. 2(b), the outer diameter (D3) is 1.00 inch and diameters D1 and D2 are 0.25 and 0.50 inch, respectively. FIG. 2(a) shows the cutting speeds at the three different diameters when the twist drill operates at 200 revolutions per minute. As illustrated in FIGS. 2(a) and (b), the cutting speeds measured at various points on the cutting edges of rotary tools will increase with the distance from the axis of rotation of the tools.
Because of these variations in cutting speed, drills and other rotary tools having a monolithic construction will not experience uniform wear and/or chipping and cracking of the tool""s cutting edges at different points ranging from the center to the outside edge of the tool""s cutting surface. Also, in drilling casehardened materials, the chisel edge is typically used to penetrate the case, while the remainder of the drill body removes material from the casehardened material""s softer core. Therefore, the chisel edge of conventional drills of monolithic construction used in that application will wear at a much faster rate than the remainder of the cutting edge, resulting in a relatively short service life for such drills. In both instances, because of the monolithic construction of conventional cemented carbide drills, frequent regrinding of the cutting edge is necessary, thus placing a significant limitation on the service life of the bit. Frequent regrinding and tool changes also result in excessive downtime for the machine tool that is being used.
Other rotary tool types of a monolithic construction suffer from similar deficiencies. For example, specially designed drill bits often are used for performing multiple operations simultaneously. Examples of such drills include step drills and subland drills. Step drills are produced by grinding one or more steps on the diameter of the drill. Such drills are used for drilling holes of multiple diameters. Subland drills may be used to perform multiple operations such as drilling, countersinking, and/or counterboring. As with regular twist drills, the service life of step and subland drills of a conventional monolithic cemented carbide construction may be severely limited because of the vast differences in cutting speeds experienced at the drills"" different diameters.
The limitations of monolithic rotary tools are also exemplified in end mills. In general, end milling is considered an inefficient metal removal technique because the end of the cutter is not supported, and the length-to-diameter ratio of end mills is usually large (usually greater than 2: 1). This causes excessive bending of the end mill and places a severe limitation on the depths of cut and feed rates that can be employed.
In order to address the problems associated with rotary tools of a monolithic construction, attempts have been made to produce rotary tools having different properties at different locations. For example, cemented carbide drills having a decarburized surface are described in U.S. Pat. Nos. 5,609,447 and 5,628,837. In the methods disclosed in those patents, carbide drills of a monolithic cemented carbide construction are heated to between 600-1100xc2x0 C. in a protective environment. This method of producing hardened drills has major limitations. First, the hardened surface layer of the drills is extremely thin and may wear away fairly quickly to expose the underlying softer cemented carbide material. Second, once the drills are redressed, the hardened surface layer is completely lost. Third, the decarburization step, an additional processing step, significantly increases the cost of the finished drill.
Thus, there exists a need for drills and other rotary tools that have different characteristics at different regions of the tool. As an example, a need exists for cemented carbide drills and other rotary tools that will experience substantially even wear regardless of the position on the tool face relative to the axis of rotation of the tool. As an additional example, a need exists for stiffer end mills wherein the increased stiffness is not achieved at the expense of tensile strength and toughness.
The present invention addresses the above-described needs by providing a composite rotary tool including at least a first region and a second region. The tool may be fabricated from, for example, cemented carbide, in which case the first region of the composite rotary tool comprises a first cemented carbide material that is autogenously bonded to a second region which comprises a second cemented carbide material. As used herein, an xe2x80x9cautogenous bondxe2x80x9d refers to a bond that develops between regions of cemented carbide or other material without the addition of filler metal or other fusing agents. The first cemented carbide material and the second cemented carbide material differ with respect to at least one characteristic. The characteristic may be, for example, modulus of elasticity, hardness, wear resistance, fracture toughness, tensile strength, corrosion resistance, coefficient of thermal expansion, and coefficient of thermal conductivity. The regions of cemented carbide material within the tool may be coaxially disposed or otherwise arranged so as to advantageously position the regions to take advantage of their particular properties. Preferred embodiments of the composite rotary tool of the present invention include twist drills, end mills, step drills, subland drills, and taps of a composite cemented carbide construction as described herein.
Cemented carbide materials generally include a binder and at least one carbide of an element selected from, for example, the group IVB, group VB and group VIB elements. The cemented carbide materials used in the present invention preferably include 60 to 98 weight percent of carbide component. The binder of cemented carbide materials typically is a metal selected from the group consisting of cobalt, nickel, iron, or alloys of these metals. The cemented carbide materials used in the present invention preferably include 2 to 40 weight percent of binder. (All weight percent ranges provided herein are inclusive of the stated upper and lower extremes unless otherwise noted.) The properties of the cemented carbide materials used in the present invention may be tailored for specific applications by varying one or any combination of the chemical composition of the ceramic component, the grain size of the ceramic component, the chemical composition of the binder, or the ratio of the binder content to the ceramic component content.
In the present invention, one or more of the ceramic component or binder chemical composition, the ceramic component grain size, and the ratio of binder content to ceramic component content differ between the at least two regions of cemented carbide material so as to advantageously select the relative properties of the two or more regions of cemented carbide material. As an example, the hardness and wear resistance of one region of a rotary tool constructed according to the invention may be enhanced relative to another cemented carbide region. In that way, the tool may be constructed so that the regions of the cutting edge experiencing relatively high cutting speeds will wear and chip at a rate similar to that of regions experiencing lower cutting speeds. Although the present invention is presented in terms of rotary tools having a finite number of regions of cemented carbide material, it will be understood that the present invention may be applied to provide rotary tools with any number of regions of cemented carbide material, each having selected properties.
The invention also includes a novel method of producing composite rotary tools of the present invention. The method includes placing a first metallurgical powder into a first region of a void within a mold. A second metallurgical powder is placed into a second region of the void of the mold. The mold may be segregated into the two (or more) regions by, for example, placing a physical partition in the void of the mold to separate the regions. The metallurgical powders may be chosen to provide, on consolidation, cemented carbide materials having the desired properties as described above. A portion of at least the first metallurgical powder and the second metallurgical powder are placed in contact within the mold. The mold is then isostatically compressed to consolidate the metallurgical powders to form a compact. The compact is subsequently sintered to further density the compact and to form an autogenous bond between the regions. Preferably, the compact is over pressure sintered at a pressure of 300-2000 psi and at a temperature of 1350-1500xc2x0 C. Subsequently, material may be removed from the sintered compact to form flutes or otherwise form a cutting edge or other features of the rotary tool.
The reader will appreciate the foregoing details and advantages of the present invention, as well as others, upon consideration of the following detailed description of embodiments of the invention. The reader also may comprehend such additional details and advantages of the present invention upon using the invention.